Moving substrates of indefinite length (viz., moving webs) can be converted in a variety of ways from one state or shape to another state or shape. Some converting processes produce considerable debris, or are carried out in the presence of airborne particulates or other contaminants, or may require a controlled environment when ordinary ambient air conditions might disrupt the converting process or pose a safety hazard. This can be a particular problem in dry converting operations, when static buildup may cause debris, particulates or other contaminants to adhere to the moving substrate. For example, optical-grade coatings on plastic films are especially sensitive to contamination, which may cause visible defects.
Typical controlled environments include clean rooms and the use of inert, low oxygen or saturated atmospheres. Clean rooms and special atmospheres require costly auxiliary equipment and large volumes of filtered air or specialty gases. For example, a typical clean room operation may require many thousands of liters per minute of filtered air.